1. Field of the Invention
The present invention relates to an image processing apparatus and, more particularly, to an image processing apparatus for producing a half-tone image from input multi-level image data (image data in which each pixel is represented by more than one bit).
2. Description of the Related Art
Laser beam printers of the type which utilize electrophotographic processes have recently received attention as one type of high speed and low-noise printer. Such a laser beam printer is typically used for recording characters, line drawings, figures or the like. Since characters, figures or the like are so called binary level images which are represented by only two states--black and white,--they need not be reproduced in halftone and the structures of printers for this purpose can therefore be made simple.
Several methods of reproducing halftone images by means of binary recording apparatus have been proposed and, for example, a dither method and a density pattern method are well known. However, as publicly known, it is difficult to realize high resolution by using printers of the type employing the dither method or the density pattern method. In such a situation, developments have recently been directed to printers capable of reproducing halftone while performing binary level recording by means of semiconductor lasers adapted to be driven with image signals whose pulse duration is varied by pulse width modulation (PWM). With the PWM method, it is possible to print out images which excel in resolution and tone representation. This PWM method is indispensable to, particularly, color-image printers.
However, the laser beam printers utilizing the PWM processes offer various types of new problems.
One problem, although inherent in the electrophotographic process, is the instability of the density of an image printed by the electrophotographic process. Another problem resides in difficulties which may occur when the semiconductor laser is driven with pulse width modulation.
These problems are explained in detail below.
FIG. 11 shows the general structure of an electrophotographic type of printer mechanism.
The illustrated printer mechanism comprises a photosensitive drum 301 which is rotated about a shaft 309 in the direction indicated by the arrow shown on the drum, a charger 302, a developer 303, a transfer charger 304, a cleaning device 305, and an optical section disposed above the photosensitive drum 301 as viewed in the figure. The charger 302, the developer 303, the transfer charger 304, and the cleaning device 305 are disposed around the photosensitive drum 301 in the order shown in the figure.
This optical section comprises a semiconductor laser unit 306, a polygon mirror 307 which rotates at a fixed high speed, an f-.theta. lens 308, a light shielding board, and the like. An image signal is obtained by applying a PWM process to a time-series digital pixel signal which is, after arithmetic operations, output from an image reader, an electronic computer or the like, which is not shown. The image signal is then supplied to the semiconductor laser unit 306. The semiconductor laser unit 306 irradiates the polygon mirror 307 with a laser beam according to the on/off states of the image signal. Since the polygon mirror 307 is rotating at fixed high speed, the laser beam, which has irradiated one face of the polygon mirror 307, scans (exposes) the drum surface at a location between the charger 302 and the developer 303 in such a manner that the beam spot travels along the longitudinal axis of the photosensitive drum 301 from the front toward the rear as viewed in FIG. 11.
In general, the characteristics, such as exposure sensitivity, residual potential and the like, of the photosensitive drum 301 change due to aging and changes in environment. In addition, developing agents (toner or the like) in the developer 303 undergo changes in the amount of electric charge they can build up or retain or the like, and such a change seriously affects development density. This is the problem of density instability which is inherent in the electrophotographic process, and has severe effects on the formation of a fine level of density in the PWM-type laser beam printer.
There is another problem in that, as shown in, for example FIG. 13, the relationship between the electric current and the power level P of the laser beam which are supplied from the semiconductor laser depends upon the ambient temperature.
To cope with these problems, various proposals have been made with respect to a method of providing a stable tonal image by properly controlling the above fluctuation factors. Typical known techniques disclosed in, for example, Japanese Patent Publication No. 43-6199, and Japanese Patent Laid-Open Application Nos. 53-93030 and 50-9639. Although the detailed explanation of them is omitted, even if any of these proposals is adopted, the problem of a deterioration in the image quality of a highlight portion still remains, which problem is to be solved by the present invention.
FIG. 9 is a circuit diagram showing one example of a PWM circuit, FIG. 10 is a circuit diagram of a laser driver circuit, and FIG. 12 is a timing chart showing the operation of the PWM circuit.
In FIG. 9, reference numeral 401 denotes a TTL latch circuit for latching an 8 bit image signal, reference numeral 402 a level converter for converting a TTL logic level into a high-speed ECL logic level, reference numeral 403 an ECL digital-to-analog converter, reference numeral 404 an ECL comparator for generating a PWM signal, reference numeral 405 a level converter for converting an ECL logic level into a TTL logic level, reference numeral 406 a clock oscillator for generating a clock signal 2f of frequency which is double the frequency of a pixel clock signal f, reference numeral 407 a triangular-wave-signal generator for generating an approximately ideal triangular wave signal in synchronization with the clock signal 2f, reference numeral 408 a divide-by-two frequency divider for dividing the clock signal 2f by two. Although not shown, ECL circuits are disposed at appropriate locations in order to cause the circuit to operate at high speed.
The operation based on the above construction is explained with reference to FIG. 12.
A signal a indicates the clock signal 2f, and a signal b indicates the pixel clock signal f whose period is twice that of the clock signal 2f. As illustrated, the waveforms of these signals a and b are associated with pixel signals. Within the triangular-wave-signal generator 407, the clock signal 2f is divided by two to generate a triangular wave signal c so that the duty ratio of the triangular wave signal can be kept 50%. Then, this triangular wave signal c (not indicated in FIG. 9) is converted into an ECL level (0 to -1V) to form a triangular signal d.
The image signal latched by the latch circuit 401 changes from 00H (white) to FFH (black) in 256 steps of tone representation Incidentally, "H" indicates hexadecimal notation The image signal e indicates ECL voltage levels which are obtained by subjecting the values of several image signals to D/A conversion in the D/A converter 403. For example, a period corresponding to the first pixel indicates FFH, representing a black pixel level, a period corresponding to the second pixel 80H, representing a halftone level, a period corresponding to the third pixel 40H, representing a halftone level, and a period corresponding to the fourth pixel 20H, representing a halftone level. The comparator 404 compares the triangular signal d with the image signal e to generate a PWM signal of pulse width T, t.sub.2, t.sub.3, t.sub.4, etc. This PWM signal is translated into a TTL level of 0V or 5V to form a PWM signal f, which is in turn supplied to the laser driver circuit 500.
In FIG. 10, a constant-current type of laser driver circuit is denoted by 500, and a semiconductor laser device by 501. This semiconductor laser device 501 is arranged to emit laser light when a switching transistor 502 is on and to stop the emission of laser light when the transistor 502 is off. This transistor 502 and a transistor 504 form as a pair, a current switching circuit to control, in accordance with the input PWM signal f, the on/off switching (commutation) of a constant current to be supplied to the semiconductor laser device 501. This constant current is variable and the value of the constant current is determined by converting the input 8 bit value of the laser power into an analog voltage by means of the D/A converter 503 and comparing the analog voltage with a reference voltage.
The response characteristics of this laser beam involve the following problems. Referring to FIG. 12, when it is assumed that the maximum emission time per pixel is T (in seconds), if the pulse width of a PWM signal changes between 0 to T seconds, then it is theoretically preferable that the semiconductor laser device 501 emit a laser beam for only a time period accurately corresponding to this pulse width. In practice, however, because the PWM signal passes through the semiconductor laser device 501 and its drive circuit 500, an actual signal, which serves to drive the laser device, assumes the waveform g shown in FIG. 12, with the result that the emission of the laser beam is started or stopped with a certain amount of response delay. If the pulse width is T or t.sub.2, this response delay is no problem, but, in the case of the pulse width t.sub.3, the laser beam is not perfectly on. Further, in the case of the pulse width t.sub.4, the semiconductor laser device 501 does not operate in practice. A beam effect h two-dimensionally indicates the state of emission of laser beams. Since the first pixel is black, the laser beam is on over the time period corresponding to one pixel. However, if the pulse width of the PWM signal becomes extremely short, for example t.sub.3 =10 ns, no laser beam may be emitted. Even if a laser beam is emitted, in such case it is extremely unstable in terms of the formation of an image by the electrophotographic process, and a stable density is no longer reproduced. As is apparent from the foregoing, the minimum pulse width which allows a satisfactory degree of density to be formed in tonal representation utilizing the PWM method has not yet reached a low enough limit. If this limit is t.sub.3 =10 ns, tone corresponding to all the pulse width not exceeding 10 ns (a highlight portion) will be represented in white.